In my commonly owned U.S. Pat. No. 3,562,966 I have disclosed an apparatus of this type in which a crankshaft being machined is mounted on a lathe for rotation about its main axis, each of the pins being surrounded by a jaw-shaped housing movable between guide plates which in turn are laterally slidable on rails paralleling that axis. Each housing contains three tool carriers which are angularly equispaced about the respective pin and are provided with abrasive stones in contact with the pin's surface. The tool carriers of each housing are jointly displaceable in a radial direction by intermeshing gears and racks under the control of a hydraulic servomotor.
While the housing engaging the bearing pins theoretically remain centered on the main axis and thus are held stationary during rotation of the crankshaft, the housings associated with the eccentric crankpins orbit about that axis while sliding between their guide plates with a pitman-like motion. In practice, however, the weight of the horizontally supported crankshaft causes it to bend away from the axis of rotation wherefore also the housings associated with the bearing pins are allowed a certain mobility by being flexibly or otherwise yieldably suspended. Actually, though, the resulting adaptability of the positions of the tool carriers to the excursions of the bearing pins has the drawback that surface irregularities due to manufacturing tolerances--often aggravated by an annealing or hardening treatment to which the crankshaft is subjected between coarse machining and finish-grinding--are not properly eliminated. In fact, the deviations of the pins from the axial direction tend to give rise to further surface deformations which may make the crankshaft unsuitable for high-precision operation.